The provision of tightly closed and sealed end closure flaps has been a long standing problem in the carton industry, particularly where powdered or granular materials are being packaged. One of the problems is to achieve a tight seal between the juxtaposed closure flaps themselves. In particular, the innermost or first folded pair of flaps are unsupported from inside the carton and hence tend to flex or bend inwardly, resulting in oily partial contact between the folded flaps. Another problem is encountered at the corners of the end closures where so-called "pin-holes" or interstices are formed as the flaps are infolded and sealed, due in part to the fact that the cartons are not truly in square condition when erected, thereby causing the flaps to be out of accurate registry relative to each other.
Numerous constructions have been proposed to provide tightly sealed end closures. From the standpoint of effecting a tight seal between the juxtaposed closure flaps, it has hitherto been proposed to fold the innermost closure flaps upon themselves and provide extensions or tabs on their end edges which are adhesively secured to the inner surfaces of the adjoining carton body walls, thereby providing a truss-like arrangement in which the innermost flaps are prevented from bending inwardly or from twisting, thereby permitting a tight seal to be effected. Such arrangement requires additional boxboard for forming the attachment flaps together with an additional folding and gluing operation. Another approach to the problem is the utilization of interlocking end closure flaps in which the innermost flaps are configured to engage in slots in the next to be folded flap, the interlocking of the several flaps providing support for the sealing pressure applied to secure the flaps together. Such arrangements also insure that the carton body walls will be square and the flaps in accurate registry, thereby further enhancing the tightness of the end closure. While these arrangements provide improved registry and sealing of the flaps, they require extremely accurate infolding of the flaps if proper registry and the interengagement of the flaps is to be achieved, which is difficult to accomplish in a high speed folding and gluing operation.
Numerous expedients have also been proposed to enhance sift-proofness, such as the provision of so-called "VanBuren" ears, which comprise extensions usually connected to the third flap to be infolded, the extensions being folded and adhered to the marginal end edges of the carton body walls. Such extensions, while increasing the sift-proof character of the carton, require additional folding and gluing operations as well as additional boxboard which add to the cost of the cartons.
Another approach to providing a sift-proof end closure is taught in commonly owned U.S. Pat. No. 3,746,244, in the name of one of the present inventors, Frank D. Bergstein, entitled "Sift-Proof Carton Construction," issued July 17, 1973. In accordance with this patent, a plurality of the carton end closure flaps are provided with mating sets of ridges or grooves which interengage when the flaps are juxtaposed and sealed together, the interengaging sets of ridges or grooves providing barriers acting to prevent the sifting and leakage of powdered or granular materials between the closure flaps. The teachings of this patent may be utilized in conjunction with the present invention to further enhance the sift-proof character of the cartons.